The rotor of a rotary machine is rotatably supported by specific devices; in particular, the shaft of the machine is supported by one or more bearings.
FIG. 1 schematically shows a turbomachine 100 comprising a stator 110 and a rotor 120. For example, the rotor 120 has a rotary shaft with a first shaft end-portion 121 protruding from a first side and a second shaft end-portion 122 protruding from a second side, and the stator 110 has a first fluid bearing 111 rotatably supporting the first shaft end-portion 121 and second fluid bearing 112 rotatably supporting the second shaft end-portion 122.
There are several types of “fluid bearings” (also known as “fluid-film bearings” that can be broadly classified into two types: “fluid dynamic bearings” and “hydrostatic bearings”): plain bearings, lemon bearings, tilting-pads bearings, etc.
FIG. 2 schematically shows a plain fluid bearing system 200 according to the prior art. It comprises a rotary shaft 210 (partially shown in FIG. 2) with a journal 211 (corresponding to an axial portion of the shaft that is delimited by two dotted lines in FIG. 2), and a plain fluid bearing 220; the journal 211 is located inside the bearing 220. The bearing 220 has a cylindrical bearing statory pad 221 (such “pad” is often called “bush” due to its cylindrical shape) around the rotary journal 211, and there is a small gap between the pad 221 and the journal 211 around the journal 211. During rotation of the shaft 210, a lubricant fluid LF is injected between the pad 221 and the journal 211 so to avoid contact and reduce friction; the lubricant fluid LF typically flows from the middle (sometimes the center as in FIG. 2) of the pad 221 to the two sides of the bearing 220.
FIG. 2 shows a theoretical (that may be considered ideal) situation wherein the axis of the shaft 210 and the axis 230 of the journal seat of the bearing 220 coincide; in this case, the gap between the pad 221 and the journal 211 is uniform all around the journal 211.
Anyway, in a rotary machine during rotation of the shaft 210, the two axes do not coincide: they may be distant and/or inclined between each other.
By way of example, FIG. 3 shows four successive positions of the journal 211 inside the pad 221 as the shaft 210 rotates about its axis; the journal 211 makes a rotation movement about its axis and an orbital movement about the axis 230 of the bearing; starting from the position in FIG. 3A, the journal makes a rotation movement of 90° clockwise and an orbital movement of 90° clockwise and reaches the position of FIG. 3B, then the journal makes a rotation movement of 90° clockwise and an orbital movement of 90° clockwise and reaches the position of FIG. 3C, then the journal makes a rotation movement of 90° clockwise and an orbital movement of 90° clockwise and reaches the position of FIG. 3D.
In this case, the gap between the pad 221 and the journal 211 is non-uniform; in particular, if a point A on a diameter D of the journal 211 is considered, the distance between the point A and the pad 221 remains the same (or does not change much) at any time; this means that the temperature of the journal in the region of point A will be higher than the temperature in e.g. an opposite region of the journal.
FIG. 4 shows an exemplary simplified temperature plot along diameter D of the journal 211: at a first end E1 (close to point A) of the diameter D there is a high temperature T1, at a second end E2 (remote from point A) of the diameter D there is a low temperature T2; this temperature plot is a perfectly straight segment; more realistically, the temperature plot is a slightly curved segment. Such non-uniform temperature distribution inside the journal causes bending of the shaft at the journal and synchronous rotor vibrations, i.e. the so-called “Morton Effect”; under certain conditions, especially in high-speed turbomachines, it can lead to synchronous rotor instability.
In order to overcome such problem, document WO2015002924A1 teaches to arrange a tubular body around the shaft at the journal; the tubular body comprises a thermal barrier that absorbs at least a portion of heat generated by the rotation of the shaft. In this way, non-uniformity reduction depends on the width and the material of the thermal barrier.